basal metabolli rate (BMR): simple definition: Calories we spend without doing any exercise.

Our basal metabolli rate (BMR) decrease over time, it decrease about 100 calories every 10 years.
Example of X guy
20 years old BMR 2200 calories per day
30 years old BMR 2100
40 years old BMR 2000
50 years old BMR 1900
60 years old BMR 1800

That is the equivalent of decreasing your basal metabolic rate to about 10 calories per month. While the BMR decrease 100 calories per year the average american reduce his calorie intake in a much slower pace. That X guy 60 years old spending 1800 calories in his BMR is now eating 1810 calories per day. It means that will have an extra 10 calories to store as fat in a daily basis. So It would be 300 per month and about 3000 per year.
For each 3000 extra you gain one pound, for each 3000 calories you burn you lose one pound.

This is not a exact number as we also exercise much less as we age.

By the way, this article was made by probettor, you wont find it any where else. Therefore is not FDA approved. I just calculated that to prove wrong to some people blaming age as the cause of them being fat. Yes, the age factor can be corrected with just a 10 calories cut per day and some light exercise.